This invention pertains generally to the field of crystalline metal oxide films, optical elements formed of such materials, and methods of producing such materials.
Ferroelectrics such as lithium niobate (an oxide) possess a large non-resonant second-order optical nonlinearity which makes such materials useful for fabrication of a variety of optical and opto-electronic devices. Examples include optical switches and modulators, frequency shifting devices, polarized controllers, pulsed waveguide lasers, surface-acoustic-wave filters, and acousto-optic devices. These materials also often possess additional useful properties, such as piezoelectric, elasto-optic, and pyroelectric effects. Conventionally, such devices are fabricated from the bulk crystal material (typically a wafer about 0.5 to 1 mm thick), although most devices use only a small fraction of the surface volume of the material. Because these oxides tend to be chemically very inert, there are only a very limited number of surface modification tools (e.g., thermal diffusion) that can be used for fabrication purposes. It would be desirable if it were possible to deposit thin films of the ferroelectric materials on a substrate while controlling the composition and purity of the deposited materials. It would also be desirable if it were possible to deposit the film in a form which can be easily etched or ablated, permitting the fabrication of photolithograhically defined two-dimensional and three-dimensional structures on a planar substrate.
Numerous attempts have been made to grow crystalline LiNbO3 and other ABO3 ferroelectrics (where A and B are other metals) on various substrates. LiNbO3 thin films, for example, have been grown on semiconductors (e.g., Si and Ge), on dielectrics (e.g., MgO and Al2O3) and on ferroelectrics (e.g., LiTaO3 and LiNbO3 itself). In general, the objective of such deposition processes is to produce a crystalline thin film, since the crystalline form of the material usually has the best optical and electronic film qualities (e.g., optical transparency and nonlinear properties). Crystalline forms (e.g., single crystal textured, or polycrystalline) of these materials, however, etch very slowly with etchants current available. For example, a 50% aqueous solution of HF will have a negligible effect on single crystal LiNbO3, and reactive ion etching (RIE) using CCl2F2:Ar:O2 results in only about 3 xcexcm/h etch rate. These etch rates are comparable to the etch rates for the masking materials that are used, making high resolution geometries essentially infeasible and resulting in very rough sidewalls with large optical losses. See J. L. Jackel, et al., xe2x80x9cReactive Ion Etching of LiNbO3,xe2x80x9d Applied Phys. Lett., Vol. 38, 1981, pp. 970 et seq.
Among the devices that utilize LiNbO3 are traveling wave electro-optic modulators. LiNbO3 traveling wave modulators are currently formed utilizing a LiNbO3 substrate containing a Mach-Zehnder waveguide geometry, a buffer layer (a thin dielectric film such as SiO2 isolating the light in the waveguide from the metal electrodes), and metal electrodes in the form of a microwave strip line. State of the art commercial traveling wave modulators (TWMs) using these structures have a 7 GHz bandwidth (corresponding to 10 Gb/s maximum transmission rate for non-return to zero (NRZ) coding) and an operating voltage at the maximum speed of Vxcfx80@7 GHz=6 volts. At 40 Gb/s (30 GHz bandwidth, NRZ), numerical simulation shows that the conventional LiNbO3 TWM requires a drive voltage of about 9 volts with an electrode length L=1.6 cm and thickness te=30 xcexcm. However, the available gallium arsenide drive electronics at this bit rate has a maximum voltage swing of about xc2x14.5 volts. Thus, the conventional TWM structure would theoretically be capable of attaining the 40 Gb/s bit rate, but there is no margin of error to allow for processing variability. To account for thermal voltage degradation and process variations in the electronics, a margin of error of about 10% must be allowed (i.e., the TWM must be capable of operating at xc2x14 volts).
Noguchi, et al. (xe2x80x9cA Broadband Ti:LiNbO3 Optical Modulator with a Ridge Structure,xe2x80x9d J. of Lightwave Technology, Vol. 13, No. 6, June 1995, pp. 1164-1168) have shown that etching 3-4 xcexcm deep ridges in the LiNbO3 above the Mach-Zehnder waveguides produces a better overlap between the optical and microwave fields, thereby allowing the drive voltage to be reduced. However, difficulties are encountered in making commercial devices having such structures because, as noted above, the etch rates of crystalline LiNbO3 are very slow. The resulting surfaces are rough, significantly increasing the waveguide""s propagation loss. In addition, the reliability of devices made using present etching techniques is questionable. A variation of this approach is shown in U.S. Pat. No. 6,172,791 to Gill, et al., in which ion implantation is used to allow etching at an angle to form ridges with reentrant sidewalls to further shape the electric field in the ridges.
In accordance with the present invention, metal oxide films, in particular lithium niobate, are formed for applications in electro-optic and optical systems. The present invention may be incorporated into the fabrication of lithium niobate traveling wave modulators (TWMs) by defining topological features in the deposited lithium niobate film that allow it to function at higher bit rates than conventional devices.
A lithium niobate optical element can be formed in accordance with the invention with a substrate, such as crystalline lithium niobate, having a top surface, a layer of crystalline lithium niobate formed on the top surface of the substrate, the layer having a thickness of at least two microns, and at least one trench in the layer of lithium niobate which is at least one micron deep. The trench may be filled with a material, such as silicon dioxide, using the same mask used to define the trench, producing a region with a dielectric constant differing from that of the surrounding layer. Further, metal films may be formed over a trench filled with a low dielectric constant material such as photoresist or SiO2. If desired, the low dielectric material may subsequently be dissolved (to further reduce the dielectric constant to that of air) by leaving the ends of the trench uncovered by the metal film and therefore accessible to a solvent. A travelling wave electro-optical modulator may utilize such structures by incorporating a waveguide in the layer of lithium niobate which has an input path, first and second arms that split from the input path, and an output path, the first and second arms coupled to the output path, and electrodes above the layer of lithium niobate typically arranged as a microwave stripline. Trenches are formed in the lithium niobate layer on either side or on both sides of the two waveguides and parallel to them. The depth and width of the trenches and their positions with respect to the waveguides may be selected to optimize the performance of the TWM. In particular, the trench geometries can be used to equalize the microwave and optical effective indices, Nmxe2x88x92No=0, and simultaneously to maintain an acceptable electrical impedance Zxcx9c50 ohms or larger. The trenches also serve to concentrate the electric field from the electrodes, reducing the required operating voltages. The use of several trenches, each of which may have different widths (typically 2-10 xcexcm), provides the necessary degrees of freedom to optimize these parameters even for thick electrode structures (which thus have low resistance). Unlike prior vertical walled or angled walled ridge structures, the flexibility of the trench geometry in accordance with the present invention (depth, width, position) allows broad tunability of the TWM parameters and does not require that the trenches be placed close to the waveguides (which is known to cause optical loss due to scattering).
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.